The Global Positioning System is a space-based radio navigation system operated by the US Air Force for the United States Government. GPS was originally developed as a military force enhancement system and continues to play this role. However, GPS has also demonstrated a significant potential to benefit the civilian community in an increasingly large variety of applications. In an effort to make the service available to the greatest number of users without adversely effecting national security interests two GPS services are provided. The Precise Positioning Service (PPS) is available primarily to the military of the United States and its allies for users properly equipped with PPS receivers. The Standard Positioning Service (SPS) is designed to provide a less accurate positioning capability than PPS for civil and all other users throughout the world.
In high precision GPS applications, it is desirable to be able to account for delay of the GPS signals caused by the ionosphere. For example, in an avionics environment utilizing GPS signals to determine the relative positions and velocities of aircraft, reduction of the error in the GPS signal increases the accuracy of the aircraft tracking data, thereby providing increased safety and reduced likelihood of collision. Thus, elimination of error in the GPS signal caused by the ionosphere to improve the accuracy of the GPS signal is highly desirable.
For each satellite in the global positioning system (GPS), the total signal comprises two transmission signals, the L1 signal having a carrier frequency of 1.57542 GHz, and the L2 signal having a carrier frequency of 1.2276 GHz. Both the L1 and the L2 signals are biphase modulated by several digital signals. The high precision code (P) is one modulating signal which has a code rate of 10.23 MHz. Another signal is an optional encoding encrypting code (E) which is utilized by the Department of Defense (DOD) to encrypt and deny unauthorized access to the L2 GPS signal. The E code has been reported to have a bandwidth of approximately 500 kHz in the open literature.
The United States Government defines the GPS Standard Positioning Service (SPS) as a position and timing service provided on the GPS L1 frequency. The GPS L1 frequency, transmitted by all GPS satellites, contains a coarse acquisition (C/A) code and a navigation data message. The GPS L1 frequency also contains a precision (P) code that is reserved for military use and is not part of the SPS. The P code can be altered without notice and will not normally be available to users who do not have valid crypto keys. GPS satellites also transmit a second ranging signal known as L2. This signal is not part of the SPS, although many civil receivers have incorporated technologies into their design that enables them to use L2 to support two-frequency corrections. SPS performance standards are not predicated upon use of L2.
The GPS Block II/IIA satellite constellation nominally comprises 24 operational satellites. Each satellite generates a navigation message based upon data periodically uploaded from a control segment which it then adds to a 1.023 MHz Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code sequence. The satellite modulates the resulting code sequence onto a 1575.42 MHz L-band carrier to create a spread spectrum ranging signal which it then broadcasts to the user community. This broadcast is referred to as the SPS ranging signal. Each C/A code is unique and provides the mechanism to identify each satellite in the constellation. The GPS satellite also transmits a second ranging signal known as L2 that supports PPS user two-frequency corrections. L2, like L1, is a spread spectrum signal and is transmitted at 1227.6 MHz.
The SPS signal, or SPS ranging signal, is an electromagnetic signal originating from an operational satellite. The SPS ranging signal consists of a Pseudo Random Noise (PRN) Coarse/Acquisition (C/A) code, a timing reference and sufficient data to support the position solution generation process.
The SPS ranging signal measurement is the difference between the ranging signal time of reception (as defined by the receiver's clock) and the time of transmission contained within the satellite's navigation data (as defined by the satellite's clock) multiplied by the speed of light. The SPS ranging signal measurement is also known as the pseudo range. The estimated geometric range is the difference between the estimated locations of a GPS satellite and an SPS receiver.
Navigation data is data provided to the SPS receiver via each satellite's ranging signal, containing the ranging signal time of transmission, the transmitting satellite's orbital elements, an almanac containing the abbreviated orbital element information to support satellite selection, ranging measurement correction information, and status flags. The position solution is the use of ranging signal measurements and navigation data from at least four satellites to solve for three position coordinates and a time offset.
Dilution of Precision (DOP) describes the magnifying effects on GPS position error induced by mapping GPS ranging errors into positions through the Position Solution. The DOP may be represented in any user local coordinate desired. Examples are HDOP for local horizontal, VDOP for local vertical, PDOP for all three coordinates, and TDOP for time. The Position Solution is the use of ranging signal measurements and navigation data from at least four satellites to solve for three position coordinates and a time offset.
The GPS signal consists of two synchronous transmissions on carrier frequencies of 1.57542 GHz and 1.2276 GHz, known as Link 1 (L1) and Link 2 (L2), respectively. Both the L1 and the L2 signals are biphase modulated by digital symbols with amplitudes of .+-.1. The digital signal sequences consist of a 10.23 Mbps high precision ranging code (P code - unclassified), a 1.023 Mbps clear/acquisition ranging code (C/A code), a 50 bps navigation information signal, and a classified encryption code (E code) overlaid on the P code. The E code forms a signal known as the Y code (Y=P.sym.E). The Y code provides GPS with anti-spoofing (A/S) capability and can be enabled or disabled by the GPS master control. Only authorized receivers have access to the Y code. Current operational policy by the DOD is to keep the Y code active at all times.
The L1 signal contains in-phase and quadrature components. The in-phase modulation is E.sym.P.sym.D and the quadrature modulation is generated by C/A.sym.D. The signal broadcast from the satellite is represented as: ##EQU1## where i denotes the i.sup.th satellite in the GPS constellation and P is the signal power.
The L1 1.023 Mbps C/A code repeats every 1 ms and provides a single frequency, low accuracy navigation signal for the Standard Position Service (SPS). Commercial users have access to the C/A and P codes. The C/A code is used by receivers to derive a hand-over word acquisition of the P code. Each P code has a repetition period of one week.
The L2 signal is modulated by either the P (or Y) code or C/A as directed by the GPS master control segment. Normal operation is selected to be P code modulation with the A/S feature enabled on L1 and L2 (Y code). The low rate navigation data may or may not be transmitted on L2 as selected by ground command. If the data message is transmitted and A/S is enabled, the L2 signal transmitted by the i.sup.th satellite is represented as: ##EQU2##
The low data rate navigation signal consists of orbital parameters and clock correction information plus other information. The DOD has implemented a feature known as selective availability (S/A) that degrades the accuracy of the navigation solution for unauthorized receivers. Authorized receivers can completely remove the effects of S/A.
The unauthorized user will not have access to the encrypted P code and cannot access the algorithm to remove the S/A degradation. The effects of S/A can be mitigated by differential GPS (DGPS) measurements and broadcast of the pseudo-range differential corrections provided the update rate is sufficiently high.
A class of algorithms known as codeless techniques can be used to measure the ionospheric delay. These algorithms are known as codeless because they do not require the knowledge or generation of the encrypted ranging codes. The codeless measurement is made by performing a cross correlation of the L1 and L2 signals. A correlation peak is detected when the ionospheric delay is equal to the delay of the L1 signal path. This technique works only because the modulation and encryption codes on both frequencies are identical and synchronous.
However, the primary disadvantage of this technique is the large SNR penalty incurred when the cross correlation must be done in the full bandwidth of the P code. The GPS signals are received well below the thermal noise. Correlating the L1 and L2 signals in the full P code bandwidth requires integration times on the order of several minutes to achieve acceptable pre-detection signal to noise ratios under weak signal conditions.
Additional disadvantage may be encountered in the use of codeless algorithms. Although ionospheric dynamics typically have time constants on the order of tens of minutes, measurements of the ionospheric delays of satellites with low elevation angles may be difficult or impossible to achieve with a high degree of accuracy. This is due to two reasons. First, the satellite signal power is very low when viewed at the horizon. Second, the signals typically experience higher dynamics at low elevation angles due to satellite and receiver geometry changes and longer ionospheric propagation path lengths. The longer integration times required under these conditions may distort dynamic information in the signal that users require for accurate and continuous correction for ionospheric delays.
It is known that the L1 and L2 signals are synchronous with respect to each other, including corresponding E codes. Since the signals are synchronous, the signals may be cross correlated with one another by utilization of a semi-codeless technique, controlling the relative delay of the signals with respect to each other and measuring the resulting correlation value. This cross correlation is performed in the reduced bandwidth of the E code. The relative delay is proportional to the ionospheric delay when the cross correlation has reached a maximum value.